1. Field of the Invention
This invention relates generally to communication systems and more particularly to systems utilizing modulated reflected energy for communicating information. Although embodiments of the invention find utility in many diverse applications, they are particularly suited for use in light aircraft identification systems for facilitating the control of airport traffic.
2. Description of the Prior Art
Prior art systems are known which enable ground stations to interrogate and ascertain the identity of an aircraft. Such systems normally use both ground based and airborne transmitters and receivers. In operation, the ground based transmitter would transmit an interrogation signal which would be recognized by the airborne receiver and cause the airborne transmitter to transmit identification information back to the ground based receiver. Although such systems normally operate satisfactorily, they usually require somewhat heavy and expensive airborne equipment and thus are unsuitable for light aircraft. Moreover, since the airborne equipment normally utilizes many active components that may require considerable electrical power, its reliability is often less than desired. Furthermore, since the aircraft has no prior knowledge as to the direction from which the interrogation may come, it must necessarily reply in an omnidirectional pattern which makes the system vulnerable to spoofed replies from unidentifiable or unfriendly aircraft.